1. Field of the Invention
This invention relates to a projection device for projecting a mirror image or inverted image of an object upon a plane of projection, and more particularly to a projection device suited for use with a compact slit exposure type copying machine having an image transfer process.
2. Description of the Prior Art
In the projection device of the copying machine or the like according to the prior art, the projection lens system thereof has comprised an ocellar lens consisting of a single lens, but this has suffered from a problem that the conjugate from the object surface to the image plane is long due to the limited angle of view taking the depth of focus or the like into consideration and necessarily the device becomes bulky. To overcome such problem and to make the size of the entire copying machine compact, there is known a projection device in which the projection lens system comprises an ommateal lens having the same angle of view as the conventional one and consisting of a plurality of lenses and the individual lenses have the transmitting function of rendering part of an object to be projected into part of a corresponding image so that the projected image of the entire object is formed on a plane of projection by the entire lens system.
U.S. Pat. No. 3,584,952 and U.S. Pat. No. 3,592,542 disclose projection devices in which a plurality of lens systems are arranged along the lengthwise direction of the slit portion and by these lens systems, the band-like area of an original to be copied is projected upon a photosensitive medium as the synthesis of part images to thereby form an entire image. By making the lenses into an ommateal lens, the effective diameter of each lens is reduced and the conjugate corresponding to the same angle of view becomes shorter, thus making the entire device compact. That is, in the projection device shown in U.S. Pat. No. 3,584,952, a lens system comprises three front, intermediate and rear lenses arranged in the direction of the optic axis so that by the front lens, an intermediate image corresponding to a part of an original is formed on the intermediate lens disposed between the front lens and the rear lens and this intermediate image is formed as the final image on a photosensitive medium by the rear lens. This intermediate lens performs the function of a field lens and has nothing to do with the original projecting function. However, this field lens has an important character in that it maintains uniform the brightness of the projected image formed on the photosensitive medium. Likewise, in U.S. Pat. No. 3,592,542, a lens system comprises three sets of front, intermediate and rear lenses, each set comprising two lenses. In this device, three lenses or three sets of lenses are arranged in the direction of the optic axis and the device has a disadvantage that setting or otherwise adjusting the mutual lenses without eccentricity is difficult.
On the other hand, a lens having a great length in the direction of the optic axis thereof as compared with the effective diameter thereof is disclosed in British Pat. No. 954,629 whose complete specification was issued on Apr. 8, 1964.
However, the lens shown in this British Patent is not a telecentric lens system, and is not of the type in which an intermediate image is formed between a first lens and a second lens and in which a predetermined light intensity distribution on the plane of projection is controlled. The lens disclosed in the British Patent is one in which the image of a part of an object is projected by one lens system and this patent does not disclose the technique of synthesizing individual part images by a lens array construction.
Japanese Patent Publication No. 30787/1970 discloses an example of the projection lens system which synthesizes part images by a lens system array and which adopts the telecentric arrangement. However, this projection lens system is a conventional lens system and is not a lens having a great length in the direction of the optic axis thereof as compared with the effective diameter thereof.
Our co-pending U.S. Application Ser. No. 889,404 , abandoned, discloses a construction which has array-like projecting optical system comprising a plurality of lenses and in which each of the projecting optical system has a first and a second lens between which the intermediate image of a part of an original is formed, the first and second lenses having a considerably great length in the direction of the optic axis thereof as compared with the effective diameter thereof, and in which the first lens is an emergence side telecentric system and the second lens is an incidence side telecentric system. However, a feature of the invention disclosed in said Ser. No. 889,404, abandoned, is that the light intensity distribution of the final image plane corresponding to the effective object field is controlled not by lens aperture eclipse but by the field stop in the intermediate image plane portion. By controlling the light intensity distribution by the field stop so that a uniform light intensity distribution is provided in the image plane area and by causing the light intensity distributions of adjacent element lens systems in the image plane area to be superposed upon one another, the uniformity, in the slit area, of the exposure amount distribution time-integrated in the original scanning direction is improved. However, there is a problem that if there is an error in the setting of the field stop, the uniformity of the exposure amount distribution is greatly destroyed. Further, the provision of the field stop itself is cumbersome. In view of these prior arts, the present invention provides an entirely novel projection device.
Optical fiber (tradename: Cellfock) having an image forming action in which the refractive index is gradually decreased from the radially central portion thereof toward the marginal portion thereof is known in Japanese Patent Publication No. 28058/1972, whereas the bar lens according to the present invention has a refractive index uniform over the entire lens.